1. Field of the Invention
The present invention relates to an image forming system, a print control method and a control program for a printing apparatus. The present invention is particularly appropriate for an image forming system that employs an ink jet printing apparatus.
2. Description of the Related Art
As an ink supply system for supplying ink a printing head applied to an ink jet printing apparatus, there has been one which has a reservoir for reserving a predetermined amount of ink to be supplied to the printing head, and in which the ink is supplied from an ink supply source to the reservoir when necessary. Such an ink supply system is hereinafter called an on-demand supply system. The ink supply source used for this system is referred to as a main tank or a first ink tank, and the reservoir for retaining a predetermined amount of ink is referred to as a sub-tank or a second ink tank. For example, while the on-demand supply system applied for a serial scan type ink jet printing apparatus has a comparatively small sub-tank and a printing head mounted on a carriage, the on-demand supply has a comparatively large main tank located at a location other than on the carriage of the printing apparatus. Further, the supply system is so constituted that as an amount of ink in the sub-tank is reduced, the ink is replenished from the main tank to the sub-tank at an appropriate timing. Furthermore, a constitution is adopted, in which, during main scanning, by separating an ink supply path between the main tank and the sub-tank spatially, or by closing an ink channel therebetween by using a valve, for example, the first and the second ink tanks are fluidically isolated.
As a method for controlling a ink replenishment timing in the ink j et printing apparatus adopting such an on-demand supply system, there has been one disclosed in Japanese Patent Application Laid-Open No. 7-32606 (1995). According to this method, the number of droplets (dots) are counted based on image data that has been received by a printing apparatus (an ink jet printer) prior to printing. In accordance with the resulting count value, a predicted amount of ink to be used is calculated, and the calculated value is compared with the amount of ink currently remaining in a sub-tank. When the amount of ink in the sub-tank is smaller, ink is supplied (the sub-tank is refilled).
According to a control method disclosed in Japanese Patent Application Laid-Open No. 2002-59569, before compressed image data is expanded for printing, the amount of ink required for the printing is predicted based on the compression parameters of the compressed image data. Then, the predicted amount of ink is compared with the amount of ink currently remaining in a sub-tank, and when the amount of ink remaining in the sub-tank is smaller, ink is supplied.
According to a method disclosed in Japanese Patent Application Laid-Open No. 7-32606 (1995), since the number of droplets for all the image data to be printed by the printing apparatus are added up, the image data must be expanded from various file forms to obtain a printable form before the addition-up of the number of the droplets. Thus, a considerable period of time is required. Further, in order to increase an expansion process speed, there has been either a method of employing a high-speed CPU or a method of executing a parallel process. Neither of these, however, is a preferable measure, because these methods cause arise in the manufacturing costs for a printing apparatus, a complicated configuration thereof and am increase in a size thereof. Furthermore, since data transmitted by an apparatus that serves an image data supply source to the recoding apparatus must once be temporarily expanded in a memory, a large capacity memory is also required. This factor also greatly affects the manufacturing costs and the size of the main body of the printing apparatus.
Small size and low price tend to be desired for a serial scanning printing apparatus. Generally, therefore, instead of a large memory being mounted, a memory buffer having a comparatively small capacity is provided, and a configuration which performs the following processes is employed.
These processes include:
receiving compressed data;
initiating data printing when a predetermined amount of data has been received and expanded in a buffer of a main body;
when printing for one scanning has been completed, releasing the buffer in which data for the pertinent scanning has accumulated; and
expanding, in the released buffer, newly received compressed data.
According to this printing apparatus, the number of all the dots to be printed on a current page is not determined until printing has completed. The method described in Japanese Patent Application Laid-Open No. 7-32606 (1995), therefore, in which the ink to be supplied to the sub-tank is determined after all the data for a page to be printed have been developed, is not a practical resolution.
According to the method described in Japanese Patent Application Laid-Open No. 2002-59569, before the expansion of all image data that are compressed by analyzing the image data parameters, the amount of ink required to print all of the image data is predicted. Thus, the predicted amount may differ from an amount of ink actually required for printing. During printing, no more ink remains in the sub tank (the ink is exhausted), a fuzzy image may be output. In order to avoid this phenomenon, when the maximum required amount of ink is predicted, the ink refilling operation from the main tank to the sub-tank is performed frequently, even though the ink sufficient for recoding remains in the sub-tank. Accordingly, the printing throughput is deteriorated. Especially when addresses are printed on multiple cards and envelopes, and when characters, such as for documents, are printed on plain paper, the amount of ink required for printing varies greatly in accordance with the type, the size, the interval and the number of characters to be printed. Therefore, it is difficult to accurately predict the amount of ink actually required for printing. This is true because, simply speaking, between a document consisting of one character and a document consisting of 100 characters, there is a difference of about 100 times in the amount of ink required for printing.